Substrate bending stiffness measurement method and system

ABSTRACT

A method for measuring substrate bending stiffness and thereby basis weight on a real time basis. Provided is a corrugator having a plurality of parallel ribs, with one or more sheets of the substrate provided below the corrugator wherein a predetermined gap exists between a topmost sheet of the sheets and the corrugator. A vacuum is applied between the corrugator and the topmost sheet, wherein the vacuum is sufficiently large to raise the topmost sheet, thereby deflecting and bending it into a profile corresponding to the arrangement and size of the corrugator ribs and bending stiffness of the substrate. One or more sensors are provided for measuring the deflection of the topmost sheet. The vacuum, an air knife output and/or a fluffer output are then adjusted according to predetermined rules and the measured deflection.

[0001] This is a divisional of U.S. application Ser. No. 10/440,696filed May 19, 2003, which claims priority from U.S. application Ser. No.10/041,047, filed Jan. 7, 2002, now U.S. Pat. No. 6,581,456, issued Jun.24, 2003 by the same inventors and same title, and claims prioritytherefrom. This divisional application is being filed in response to arestriction requirement in that prior application.

[0002] Especially with the advent of high speed xerographic reproductionmachines wherein copiers or printers can produce at a rate in excess ofone hundred and twenty pages per minute (PPM), there is a need for sheethandling systems to feed paper or other substrate through each processstation in a rapid succession in a reliable and dependable manner inorder to utilize the full capabilities of the reproduction machine.These sheet handling systems must operate flawlessly to virtuallyeliminate the risk of damaging the substrate and to minimize machineshutdowns due to misfeeds or multifeeds. It is in the initial separationof the individual sheets from the substrate stack where the greatestnumber of problems occur.

[0003] One of the sheet feeders best known for high speed operation isthe top vacuum corrugation feeder with front air knife. In this system,a vacuum plenum with a plurality of friction belts arranged to run overthe vacuum plenum is placed at the top of a stack of sheets in a supplytray. Several fluffers are located around the perimeter of the stack forinjecting air into the top of the stack. When vacuum is supplied to thevacuum plenum, the resulting vacuum field draws one or more sheetsagainst the friction belts. At the front of the stack, an air knife isused to inject air into the acquired sheets to separate the top sheetfrom the remainder of the sheets which then are pushed down onto thestack. In operation, the vacuum pulls one or more sheets up and acquiresthem, and then air is injected by the air knife toward the acquiredsheets to separate the top sheet. Following separation, the belttransport drives the sheet forward off the stack of sheets. In thisconfiguration, separation of the next sheet cannot take place until thetop sheet has cleared the stack. In this type of feeding system everyoperation takes place in succession or serially and therefore thefeeding of subsequent sheets cannot be started until the feeding of theprevious sheet has been completed.

[0004] A variation of the paper feeder technology described above uses areciprocating feedhead in lieu of a friction belt transport to drive thetop sheet into the paper path, e.g., U.S Pat. No. 6,264,188. At theappropriate time during the feed cycle, the feedhead moves towards takeaway rolls, carrying the acquired top sheet with it. The leading edge ofthe top sheet then enters the take away roll nip, and the take awayrolls remove the sheet from the feedhead, which then cycles back to itsoriginal position. Within the feedhead are several parallel ribs whichinduce a corrugation pattern in the acquired sheets, thus creating gapsbetween the sheets, facilitating sheet separation by the air knife

[0005] Current top and bottom vacuum corrugation feeders utilize avalved vacuum feedhead, e.g., U.S. Pat. No. 4,269,406. At theappropriate time during the feed cycle the valve is actuated,establishing a flow and hence a negative pressure field over the stacktop or bottom if a bottom vacuum corrugation feeder is employed. Thisfield causes the movement of the top sheet(s) to the vacuum feedheadwhere the sheet is then transported to the take away rolls. Once thesheet feed edge is under control of the take away rolls, the vacuum isshut off. The trail edge of this sheet exiting the feedhead area is thecriteria for again activating the vacuum valve for the next feeding.

[0006] A method for measuring substrate bending stiffness and therebybasis weight on a real time basis is provided in the disclosedembodiment. A corrugator having a plurality of ribs is provided, withone or more sheets of the substrate provided below the corrugatorwherein a predetermined gap exists between a topmost sheet of the sheetsand the corrugator. A vacuum is applied between the corrugator and thetopmost sheet wherein the vacuum is sufficiently large to raise thetopmost sheet thereby deflecting and bending it into a profilecorresponding to the arrangement and size of the corrugator ribs andbending stiffness of the substrate. One or more sensors may be providedfor measuring the deflection of the topmost sheet. The vacuum, an airknife output and/or a fluffer output may then be adjusted according topredetermined rules and the measured deflection.

[0007]FIG. 1 is one example of a feedhead corrugator and two sheets ofsubstrate prior to application of a vacuum;

[0008]FIG. 2 is the feedhead corrugator and two sheets of 300 gsm papersubsequent to application of a vacuum;

[0009]FIG. 3 is the feedhead corrugator and two sheets of 110# papersubsequent to application of a vacuum;

[0010]FIG. 4 is the feedhead corrugator and two sheets of 32# papersubsequent to application of a vacuum;

[0011]FIG. 5 is the feedhead corrugator and two sheets of 20# papersubsequent to application of a vacuum;

[0012]FIG. 6 is the feedhead corrugator and two sheets of 13# papersubsequent to application of a vacuum;

[0013]FIG. 7 is a distribution of sensor output voltages for variouspaper basis weights for a number of runs;

[0014]FIG. 8 is an alternate feedhead corrugator and two sheets of 75gsm (20# bond) paper subsequent to application of a vacuum;

[0015]FIG. 9 is a graph of bending profiles for papers of a variety ofbasis weights;

[0016]FIG. 10 is a graph of substrate deflection versus substrate basisweight; and

[0017]FIG. 11 is a representational schematic of a reprographic systemaccording to the present invention.

[0018] For a general understanding of the features of the presentinvention, reference is made to the drawings. In the drawings, likereference numerals have been used throughout to designate like elements.It will become evident from the following discussion that the presentinvention and the various embodiments set forth herein are suited foruse in a wide variety of printing and copying systems, and are notnecessarily limited in application to the particular systems shownherein.

[0019] Printing and copying systems utilizing a vacuum to acquire asheet of paper or other substrate from a stack have employed acorrugated contact surface feedhead composed of a combination of variantsized ribs to reduce the bonding forces between paper surfaces, therebyseparating sheets on the contact surface to reduce the likelihood ofremoving other sheets from the stack (i.e., to reduce multi-feeds).

[0020] It is well known in the art that there are bonding forces betweensubstrate surfaces, either due to vacuum, electrostatic, or edge weddingforces or other sources. In a vacuum feeder, to separate one sheet ofsubstrate from another, air is blown into the space between multiplyacquired sheet surfaces, so that there are essentially two steps insheet separation in a vacuum feeder: one is to generate a gap and theother to blow air into the gap. The latter function is performed by airknives. Without a corrugator, applying only a uniform vacuum to pullsheets apart is very unreliable and if more than one sheet is acquiredto a flat vacuum substrate contact surface, a serious problem occursbecause there is no meaningful force to separate the sheets acquiredexcept gravity, which will not guarantee a sheet separation. To break upthe paper bond to initiate gaps, it is beneficial to have an additionalstress acting on the substrate surfaces, and the ribs of the corrugatedcontact surface are instrumental in providing additional stress toseparate sheets of substrate. FIGS. 1 and 2 show sheet separation of a300 gsm heavyweight paper during a prefeed acquisition phase. Acorrugator 10 having ribs 12 is positioned above a stack 13 of theheavyweight paper where, for clarity, only the two topmost acquiredsheets 14 are shown at the instant before sheet deflection occurs. InFIG. 2, a vacuum applied to the open space 16 between the corrugator 10and the acquired sheets 14 causes the first sheet 18 and the secondsheet 20 to be drawn toward the corrugator while creating a gap 22between the first and second sheets. The vacuum which was used toacquire the first sheet 18 forces the sheet to conform to the ribs.Since any additional acquired sheets are not subjected to the fullvacuum from the feedhead, the additional sheets do not deform nearly asmuch as they are pulled against the ribs. The gap 22 provides a spacefor an air knife (not shown) to blow air in order to further separatethe first and second sheets, 18 and 20, causing the second sheet to fallback onto the stack 13 before forward feeding the first sheet to thenext station in the system.

[0021] A major challenge in developing any substrate handling subsystemis to accommodate a wide variety of substrates without any informationfrom the user. FIGS. 3-6 illustrate the effects of applying a constantvacuum between the corrugator 10 and the two topmost acquired sheets 14during a prefeed acquisition phase for a variety of substrates havingdifferent basis weights and bending stiffnesses. FIG. 3 shows thebending profile for heavyweight paper of a first 110# sheet 24 and asecond 110# sheet 26 and the resulting 110# gap 28. The first 110# sheet24 is observed to be in contact with seven corrugator ribs 30, 32, 34,36, 38, 40 and 42. The 110# gap 28 is also observed to be sufficientlylarge for an air knife to induce further separation of first and secondsheets 24 and 26.

[0022]FIG. 4 illustrates the bending profile for 32# paper of a firstsheet 40 and a second sheet 42 and the resulting 32# gap 44. The first32# sheet 40 is observed to be in contact with the same seven corrugatorribs 30, 32, 34, 36, 38, 40 and 42 contacted by the first 110# sheet 24.The 32# gap 48 is again observed to be sufficiently large for an airknife to induce further separation of first and second sheets 44 and 46without excessive bending of the first and second sheets 44 and 46.

[0023]FIG. 5 depicts the bending profile for a medium weight 20# paperof a first sheet 50 and a second sheet 52 and the resulting 20# gap 54.The first 20# sheet 50 is observed to be in contact with all ninecorrugator ribs including the seven corrugator ribs 30, 32, 34, 36, 38,40 and 42 and, additionally, ribs 56 and 58.

[0024]FIG. 6 shows the bending profile for a lightweight 13# paper of afirst sheet 60 and a second sheet 62 and the resulting 13# gap 64. Thetop sheet 60 now exhibits much more bending than occurred with theheavier weight sheets, however, the points of maximum separation betweenthe first two sheets 60 and 62, as observed in gap 64, advantageouslyremains near the center rib 36 which provides a consistent target areafor an air knife which will maintain good efficacy in regards todropping of the second sheet 62 back onto the stack 13.

[0025] It is evident from FIGS. 3-6 that the correct vacuum level,fluffer output, and air knife output are a function of the basis weightof the paper or other substrate being acquired by corrugator 10. Itshould be noted, however that while adjusting the vacuum according tothe bending stiffness is possible, it is sufficient to only adjust thefluffer and perhaps the air knife outputs. If the vacuum is variable, alookup table would also need to compensate for the fact that thedeflection of the top sheet is directly proportional to the appliedvacuum, otherwise a misidentification of the paper might occur in anymeasurements based on bending which take place after the vacuum ischanged. In fact, in simpler embodiments, it is not necessary to providean infinitely adjustable, or continuously variable, air knife or flufferoutput levels. This fact is evidenced by FIG. 7 which provides adistribution of sensor output voltages for a number of different runsindicating the height of a topmost sheet of paper after being acquiredby corrugator 10. To obtain this data, an analog sensor was installed incorrugator 10 to produce an output voltage corresponding to the peakheight of the first sheet of paper after being acquired to thecorrugator by a constant vacuum.

[0026] A number of runs were performed with each of selected weights ofpaper and a probability distribution was calculated for each selectedweight with the results graphed in FIG. 7. Graph segment 70 representssensor output distribution for a 51 gsm paper, segment 72 representssensor output for a 75 gsm paper, segment 74 represents sensor outputfor a 90 gsm paper, segment 76 represents sensor output for a 105 gsmpaper, segment 78 represents sensor output for a 120 gsm color paper,and segment 80 represents sensor output for a 120 gsm text paper.

[0027] It is readily apparent from FIG. 7 that there are two majorgroupings of paper weights. Segments 70 and 72 representing paperweights of 75 gsm or less and segments 74-80 representing paper weightsof 90 gsm or more. In the interest of efficiency and economy, therefore,it is possible to design, for one embodiment, a vacuum corrugated feederoptimized for two groups of paper weight ranges. Such a system can havea single switch point for selecting between one of two settings ofvacuum level, fluffer output and/or air knife output. The presentconcept facilitates this task by enabling on-line measurement of thesubstrate bending stiffness while the sheet is corrugated by a VCF(vacuum controlled feeder) feedhead. Using this information, the paperfeeder and other paper handling subsystems can be optimized for thesubstrate currently being used in real time.

[0028] For the simplest case where the system is designed for two groupsof paper weights, or bending stiffnesses, a simple optical sensor may beemployed which selects a lower vacuum level, fluffer output and/or airknife output whenever an optical line of sight is broken by the firstsheet of paper being raised above a predetermined point. Alternately, amore complicated sensor arrangement utilizing analog or digital sensorsmay be employed providing a system of continuously variable settings, ordiscrete settings in finer increments. FIG. 8 shows a corrugator 82design that is particularly effective for use with an embodiment of thepresent invention wherein the height of the first sheet 84 can bereadily detected by a sensor. Two 75 gsm sheets, 84 and 86, are acquiredto a feedhead corrugator 82 using multiple ribs. As the second sheet isnot subjected to the same magnitude of vacuum as the top sheet, thesecond sheet deflects less and begins to slip off the top sheet. Duringthis process, intersheet gaps 88 are created at the lead edge and nearthe center, which allows air to flow in between the two sheets. Thesheets then separate, with the second sheet falling back onto the stack.

[0029] The degree of deflection is also dependent on the bendingstiffness of the paper. Given the same level of vacuum, a lightweightpaper such as 16# bond will deform, or corrugate, much more than a heavypaper such as 100# uncoated cover stock. Embodiments of the presentinvention describe a method where the amount of corrugation is measuredto obtain an estimate of the basis weight of the paper.

[0030]FIG. 9 gives the top sheet deflection profiles 92 generated by aMultiple Vacuum Corrugation Feeder (MVCF) for various basis weights.Profile 94 represents 50 gsm (13# bond) paper, profile 96 represents 75gsm (20# bond) paper, profile 98 represents 120 gsm (80# offset) paper,profile 100 represents 203 gsm (110# index) paper, and profile 102represents 300 gsm (110# cover) paper. The MVCF uses several ribs tocreate multiple corrugations in the top sheet to enable betterseparation. A 2-D finite element model (a method well known in the art)is generated to determine the optimum location and size of the ribs, andthe curves given in FIG. 9 are partial results from the analysis.

[0031] Upon inspection of FIG. 9, it can be readily seen that there isan inversely proportional relationship between the basis weight and themaximum deflection of the top sheet. By placing a displacement sensorabout 20 mm from the feedhead centerline, it is possible to use themeasured vertical displacement (no corrugation to full corrugation) todetermine the stiffness of the paper. FIG. 10 indicates how thisdisplacement relates to the basis weight. Curve 104 represents thevertical deflection profile 106 versus the substrate basis weight 108.Using a lookup table, a microcontroller looks up preferred operatingparameters and adjusts the vacuum level, fluffer pressure, and/or airknife pressures to optimal values for the paper being fed. As describedabove, if only two different air knife/fluffer pressure settings arerequired, the displacement sensor may only need to be comprised of asimple on/off optical sensor and a mechanism which “breaks the beam” ofthe sensor if the sheet deflects more than a set amount.

[0032] It is to be noted that the system optionally defaults to aworst-case scenario, heavyweight substrate, before acquiring the initialsheet at the start of a run. The system then may tune itself dynamicallyin real time according to measurements of the initial and subsequentsheets of substrate. The system may perform adjustments duringacquisition of the initial sheet where system response time issufficiently small, or it may perform adjustments following acquisitionof the initial sheet. This concept facilitates providing a systemwherein no user input is required to determine the weight or bendingstiffiness of the substrate.

[0033] There have been previous inventions developed to accomplish thistask. In U.S. Pat. No. 5,138,178, Lam F. Wong et al describe aphotoelectric paper basis weight sensor which presumes that the amountof energy transmitted through the substrate is inversely proportional tothe basis weight. While this is true for uncoated papers, the higherreflectivity of coated papers and low opacity of transparencies “fool”such a system into estimating a higher or lower basis weight than whatis actually true. Also, when the bending stiffness of uncoated andcoated papers are compared, coated papers are less stiff compared touncoated papers of the same basis weight. As the embodiments describedherein measure the deflection of the paper, the bending stiffness isdirectly obtained via elastic theory. Furthermore, the bending stiffnessis a more useful quantity for optimizing paper handling subsystems thanthe basis weight. For example, it is important for the leading edge of asheet of paper to be correctly aligned when entering a process stationwhich is primarily a function of bending stiffness whenever the path isother than a straight line path.

[0034] Other methods for bending stiffness measurement include bendingthe paper a set distance and measuring the force (U.S. Pat. Nos.4,866,984 & 4,991,432) and using pressure differences to deform thesheet and then measuring the corresponding deflections(U.S. Pat. No.5,297,062). These methods, however, require more complex hardware toperform the same task and are not suited to measuring the bendingstiffness of substrate being drawn off the top of a stack at a highfrequency rate. Embodiments of the present invention may make use of anexisting feedhead to produce the deflection required to calculate thebending stiffness of the substrate. In the case described above, only anoptical sensor and a simple linkage are required.

[0035] Implementing the paper stiffness sensor enables the use of lowerpressure settings for low to medium weight papers that are most oftenused in an office setting. It also, thereby, reduces the electricalpower consumed by the air blowers, resulting in a lower operating costfor the customer. These lower pressure settings further result in theblowers producing less noise, which is also another important customerconsideration. Concepts of the present invention also act to eliminatethe potential need for the customer to indicate what type of paper iscurrently being used, thereby eliminating a source of error. It is alsonoted, as the bending stiffness more directly relates to paper handlingperformance, a product whose subsystems are optimized using thesetechniques are more robust resulting in fewer paper-related failures.Flutter problems associated with using too high air pressures for agiven paper weight or bending stiffness are also reduced.

[0036] A system using concepts described herein may be tuned to operateat a better energy efficiency by reducing the energy consumed forlightweight papers. For example, the basis weight information availablefrom the sensor or lookup table can be used for the finishing device ofa reprographic system. In a xerographic system, a fuser needs togenerate a certain amount of heat that is used to fuse the toner ontothe substrate (the paper). The thicker the sheet is, the more heat thatmust be generated because, in order to achieve the fuse temperature, thethermal capacity of the substrate must be overcome. It takes aheavyweight paper longer to heat than a lightweight paper, thusrequiring a greater quantity of heat. Without knowing the basis weightof the substrate, a worst-case setting must be used, thereby wastingenergy. The present invention lends itself to providing a self-tuningsystem whereby the amount of fuser heat generated is adjusted real timeaccording to the basis weight of the substrate where the basis weight isestimated according to the measured bending stiffness of the substrate.Other substrate handling subsystems dependent on the bending stiffnessor basis weight of the substrate may also be self-tuned without any userinput.

[0037] With attention toward FIG. 11, a reprographic system 110 is shownin schematic form that is suitable for embodiments of the presentinvention. The system includes a user interface 112, system memory 114into which are incorporated a control program 116 and a lookup table118, a vacuum source 120, a feedhead corrugator 122, a sensorarrangement 124, a substrate/paper tray 126 for a stack of substrate128, an air knife 130, a fluffer 131, a forward feeding unit 132, afinishing unit 134, a fuser 136, a fuser temperature control 138, and animage source 140. The system is shown in a representational schematicform as the components illustrated are well known in the art. The imagesource may be a scanning device or a network connection for receivingimages from a digital network. The sensor arrangement 124 may be digitalor analog and may provide a continuous output representing first sheetheight, or may provide discrete output representing one or more heightsfor the first sheet. Sensor arrangement 124 may be designed as a singlesensor or as multiple sensors. The control program is configured toutilize sensor arrangement 124 and the lookup table 118 as previouslydescribed.

[0038] While the invention has been described with respect to specificembodiments by way of illustration, it is to be understood that theappended claims cover all such modifications and changes which fallwithin the true spirit and scope of the invention.

What is claimed is:
 1. A pneumatic sheet separating and feeding methodincluding a sheet feedhead for separating and feeding the top sheet froma stack of print substrate sheets of variable sheet beam strengths bypneumatically forcing the top sheet against said sheet feedhead, whereinsaid sheet feedhead has a plurality of differentially spaced extendingribs of different rib extensions against which different deformationcorrugation shapes of the top sheet are pneumatically formed dependingon said variable sheet beam strength of the top sheet.
 2. The pneumaticsheet separating and feeding method of claim 1, wherein saiddifferentially spaced ribs of different rib lengths ribs of said sheetfeedhead are substantially parallel to one another.
 3. The pneumaticsheet separating and feeding system of claim 1, wherein said sheetfeedhead further includes sheet deformation sensing of said differentdeformation corrugation shapes of the top sheet against said sheetfeedhead depending on said variable sheet beam strength of the top sheetto provide an estimate of the sheet beam strength of that top sheet. 4.The pneumatic sheet separating and feeding system of claim 3, whereinsaid estimation of the sheet beam strength of that top sheet is utilizedto control the pneumatic sheet separating and feeding pneumatic forcelevel.